The present invention relates to an axle drive unit for a drive train, in particular for a cut-in axle of an all-wheel drive train, with an input element which is connectable to an input shaft, such as a cardan shaft, extending in a longitudinal direction, and with a power-split device, of which the input member is connected to the input element and of which the output members are connectable to driveshafts of the axle which is oriented in a direction transverse to the longitudinal direction.
An axle drive unit of this type is generally known, for example from “Das groβe Buch der Allradtechnik” [“The big book of all-wheel technology”], Jür-gen Stockmar, 1 st edition 2004, Stuttgart, page 95.
In recent years, there has been a trend, in drive trains for motor vehicles, towards all-wheel technology, specifically not only in cross-country vehicles, but also in road vehicles, such as passenger cars.
A distinction is made, in general, between a permanent all-wheel drive and a cut-in all-wheel drive. In the permanent all-wheel drive, the drive power of an engine is generally distributed to the axles (front axle and rear axle) via a longitudinal differential. In the cut-in all-wheel drive, one axle is driven constantly, and the other axle is cut in, as required.
Vehicles with a permanent all-wheel drive have disadvantages in terms of weight and efficiency on account of the comparatively complicated longitudinal differential. For this reason, nowadays, vehicles are mostly equipped with a cut-in all-wheel drive.
Cutting-in previously took place via dog clutches which could implement only two states (OPEN/SHUT). As a result, such dog clutches could, as a rule, be selected only at a standstill, which was usually sufficient for travelling across country. With the introduction of the all-wheel drive for road vehicles, cutting-in was to be possible even during travel. Furthermore, for such road vehicles, cutting-in was to take place in an as automated manner as possible, that is to say require no action by the driver. For this purpose, self-locking clutch systems for cutting in the second axle were developed. These were, in the first place, visco-clutches, such as are shown, for example, in “Das groβe Buch der Allradtechnik” [“The big book of all-wheel technology”], Jürgen Stockmar, 1 st edition 2004, page 95. The axle drive unit shown there has a hang-on clutch which is designed as a visco-clutch and which is connected on the input side to a cardan shaft and is connected on the output side, via a single angle drive, to a transverse differential of the cut-in axle.
Furthermore, it is known to arrange in the region of a cut-in rear axle, adjacently to the transverse differential, a multiple-disc clutch activated by means of a pump which is put into operation in the case of a differential rotational speed between the front axle and the rear axle. The higher the differential rotational speed is, the higher is the pressure generated by the pump.
The multiple-disc clutch is likewise a hang-on clutch and is connected between a differential cage of the transverse differential and a crown wheel of an angle drive. A drive pinion of the angle drive is connected with the cardan shaft which runs from the front axle to the rear axle.
In self-locking differentials which exert a locking action on account of a differential rotational speed between the axles, a foresighted cut-in is usually not possible.
For this reason, there is a trend towards electronically regulatable hang-on clutches which are activated by a drive dynamics controller and assist in stabilizing the vehicle in specific driving situations.
The advantage of installing the hang-on clutch in the region between the cardan shaft and an angle drive to the cut-in axle is that, although the rotational speeds to be transmitted are relatively high, the torque to be transmitted via such hang-on clutches is nevertheless correspondingly lower.
When the hang-on clutch is arranged between the crown wheel and the differential cage, the torque is higher by the amount of the step-up of the angle drive, so that the hang-on clutch has to be of a comparatively bulky design.
Concepts for hang-on axle drive units are also known in which the hang-on clutch is mounted between a driven shaft of the transverse differential of the cut-in axle and the corresponding side wheel. When the hang-on clutch is installed at this location, the clutch has to transmit only half the axle torque.
In so far as the drive train is to be used for dynamically influencing the driving behaviour about the yaw axis, active yaw systems, as they are known, are employed. Suitable axle drive units are mentioned on pages 130 and 131 of the abovementioned prior art “Das groβe Buch der Allradtechnik” [“The big book of all-wheel technology”].